The interest in deploying low-power nodes, such as pico base stations, home eNodeBs, relays, remote radio heads, etc, for enhancing the macro network performance in terms of the network coverage, capacity, and service experience of individual users has been constantly increasing over the last few years. At the same time, there is a need for enhanced interference management techniques to address interference issues caused, for example, by a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks.
In the 3rd Generation Partnership Project (3GPP), heterogeneous network deployments are defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout leading to non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adapt to the traffic needs and the environment. However, heterogeneous deployments bring also challenges for which the network needs to be prepared to ensure efficient network operation and superior user experience. Some challenges are related to the increased interference in the attempt to increase small cells associated with low-power nodes—cell range expansion. Other challenges relate to potentially high interference in uplink due to a mix of large and small cells.
In a heterogeneous deployment, according to 3GPP, low power nodes are placed throughout a macro-cell layout. The interference characteristics in a heterogeneous deployment can be significantly different than in a homogeneous deployment, in the downlink, the uplink, or both. Examples are shown in FIG. 1, where in case (a), a macro user with no access to a Closed Subscriber Group (CSG) cell is interfered by the home base station HeNB, in case (b), a macro user causes severe interference towards the HeNB, in case (c), a CSG user is interfered by another CSG HeNB, and in case (d) a UE is served by a pico cell in the expanded cell range area. Thus, in general it should be understood that a heterogeneous deployment does not necessarily involve CSG cells.
Interference Coordination in Heterogeneous Networks.
HetNet solutions for Long-Term Evolution (LTE) focus on the DL. The need for enhanced Inter-Cell Interference Coordination (eICIC) techniques for DL transmissions in such HetNets is particularly crucial when the cell assignment rule diverges from the Reference Signal Received Power (RSRP)-based approach, e.g., towards path loss-based or path gain-based approach, sometimes also referred to as the cell range expansion when adopted for cells with a transmit power lower than neighbor cells. The concept of the cell range expansion is illustrated in FIG. 2, where the cell range expansion of a pico cell is implemented using a delta-parameter, Δ.
To facilitate measurements in the extended cell range, i.e., where high interference is expected, the 3GPP standard specifies Almost Blank Subframe (ABS) patterns for eNodeBs and restricted measurement patterns for UEs. A pattern is a bit string indicating restricted and unrestricted subframes characterized by a length and periodicity, which are different for Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD), e.g. 40 subframes for FDD and 20, 60 or 70 subframes for TDD. Restricted measurement subframes are configured to allow the UE to perform measurements in subframes with improved interference conditions, which may be implemented by configuring ABS patterns at eNodeBs, and avoid measuring in high-interference conditions. Restricted measurement patterns are in general UE-specific. Three patterns are currently specified in the 3GPP standard to enable restricted measurements: (1) serving-cell pattern for Radio Link Monitoring (RLM) and Remote Monitoring and Management (RRM) measurements, (2) neighbor-cell pattern for RRM measurements, and (3) serving-cell pattern for Channel State Information (CSI) measurements.
An ABS pattern is a transmission pattern at the radio node; it is cell-specific and may be different from the restricted measurement patterns signaled to the UE. In a general case, ABS patterns are low-power and/or low-transmission activity subframes. ABS patterns may be exchanged between eNodeBs via X2, but these patterns are not signalled to the UE, unlike the restricted measurement patterns.
Transmit Activity Patterns.
ABS is one example of transmit activity patterns. The pattern is specified per cell, and in the current standard may be exchanged between eNodeBs via X2. ABS is currently defined as “a subframe with reduced power on some physical channels and/or reduced activity,” see 3GPP TS 36.423. There are other signaling enhancements of transmit activity patterns. Multi-level patterns have been described where the “level” may be associated with a decision comprising a setting of one or more parameters where the setting characterizes a low-transmission activity, and the parameters may e.g. be any of transmit power, bandwidth, frequency, subset of subcarriers, etc. This is closely related to the actively discussed currently in 3GPP reduced transmit power subframes, e.g. non-empty ABS subframes where some data transmissions may be scheduled but with a lower power. Such patterns may be signaled to the UE. Such patterns may be associated with either overall transmissions from the node or particular signal(s), e.g. positioning reference signals (PRS), or channel(s), e.g. data channels and/or control channels. It has been described that the serving network node may indicate to the UE that DL and/or uplink (UL) ABS, or any type of low interference DL and/or UL sub-frames, are configured in certain sub-frames. The UE becomes thus aware of the transmit pattern of the serving or neighbor radio node and may use this information for performing an auxiliary action, e.g., going to a sleeping mode, executing low-priority tasks, etc. Transmit activity patterns may also be signaled to other network nodes, e.g., positioning node. The patterns are not defined though for physical signals (physical signals include also reference signals), but for Physical Downlink Shared Channel (PDSCH).
Downlink Transmissions and Power Allocation.
The transmit power information may be used, e.g., for path loss estimation, channel estimation, estimation of the signal for advanced receiver techniques, e.g., interference suppression or interference cancellation, ensuring consistent signal measurements, e.g., Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ), etc. Some example applications are power control, e.g., UL power control based on DL path loss, interference coordination, e.g., determining the distance to a neighbour node or interference estimation for a cell, DL transmit power adjustment by home eNodeBs for co-channel co-existence with other radio nodes, self-organizing network (SON), positioning, e.g., using path loss for distance estimation or as a fingerprint, etc.
The power allocation approaches for different downlink signals and channels are further described. There are also some limited means of informing the UE about the DL transmit power. In general, if the measuring node is not aware of the transmit power levels, the received signal variation due to the transmit power changes is hard (if possible at all) to distinguish from fast fading and shadowing effects.
The following physical signals may be transmitted in DL with the current LTE standard: (1) reference signals such as cell-specific reference signals (CRS), Multicast-Broadcast Single Frequency Network (MBSFN) reference signals, UE-specific reference signals (DM-RS), positioning reference signals (PRS), CSI reference signals (CSI-RS), and (2) synchronization signals such as primary synchronization signals (PSS) and secondary synchronization signals (SSS). The following channels may be transmitted in DL: Physical Downlink Shared Channel (PDSCH), Physical Broadcast Channel (PBCH), Physical Multicast Channel (PMCH), Physical Control Format Indicator Channel (PCFICH), Physical Downlink Control Channel (PDCCH), and Physical Hybrid ARQ Indicator Channel (PHICH). These channels are used for different purposes e.g. to transmit data, control, broadcast information etc. Examples of data channels are Physical Downlink Shared Channel (PDSCH) and Physical Multicast Channel (PMCH). The PMCH carries data which is broadcasted to multiple users typically from multiple base stations. Examples of control channels are Physical Control Format Indicator Channel (PCFICH), Physical Downlink Control Channel (PDCCH) and Physical Hybrid ARQ Indicator Channel (PHICH). An example of a broadcast channel is Physical Broadcast Channel (PBCH).
DL Power Allocation.
UMTS provides dynamic DL power control generally based on measurements performed by the UE on UE-specific pilot signals, which are transmitted on a UE-specific DL physical channel called DPCCH. With High-Speed Downlink Packet Access (HSDPA) without a dedicated channel, the Transmit Power Control (TPC) commands sent on F-DPCH are used by the UE for downlink power control. The UE however may still use the common pilot signals, e.g. common pilot channel (CPICH), for performing channel estimation.
In LTE, there is no dynamic DL power control like there is in UMTS. However, the Base Station (BS) may still perform some sort of power adjustment, e.g. for data transmissions and for PRS, based on UE measurements like CSI reports, although the power variation may not be sufficient to maintain the channel estimation quality. Hence there are some basic means for DL power adjustment for specific channels/signals, in addition to general BS power requirements.
DL Transmit Power Parameters and Requirements.
The DL transmit power levels are typically defined by at least the following parameters:                Output power (Pout) of BS is the mean power of one carrier delivered to a load with resistance equal to the nominal load impedance of the transmitter.        Maximum total output power (Pmax) of BS is the mean power level measured at the antenna connector during the transmitter ON period in a specified reference condition.        Rated total output power of BS is the mean power for BS operating in single carrier, multi-carrier, or carrier aggregation configurations that the manufacturer has declared to be available at the antenna connector during the transmitter ON period.        Maximum output power (Pmax,c) of BS is the mean power level per carrier measured at the antenna connector during the transmitter ON period in a specified reference condition (e.g. min/max temperatures, relative humidity, etc). The reference conditions are used to verify the BS maximum output power, e.g., when testing.        Rated output power (PRAT) of BS is the mean power level per carrier for BS operating in single carrier, multi-carrier, or carrier aggregation configurations that the manufacturer has declared to be available at the antenna connector during the transmitter ON period (see Table 1). Different PRATs may be declared for different configurations.        
TABLE 1Base Station rated output power [TS 36.104, Table 6.2-1]BS classPRATWide Area BSNo upper power limitLocal Area BS≦+24 dBm (for one transmit antenna port)≦+21 dBm (for two transmit antenna ports)≦+18 dBm (for four transmit antenna ports) <+15 dBm (for eight transmit antenna ports)Home BS≦+20 dBm (for one transmit antenna port)≦+17 dBm (for two transmit antenna ports)≦+14 dBm (for four transmit antenna ports) <+11 dBm (for eight transmit antenna ports)
Typically, in normal conditions, the BS maximum output power shall remain within +2 dB and −2 dB of the rated output power. In extreme conditions, the base station maximum output power shall remain within +2.5 dB and −2.5 dB of the rated output power. Home BS output power Pout may need to be further adjusted to protect another Universal Terrestrial Radio Access (UTRA) network or Evolved-UTRA (E-UTRA) network on adjacent channel or E-UTRA network on the same channel. The transmitter OFF power is determined by requirements. The value is measured over a certain period and the spectral density shall be less than −85 dBm/MHz [36.133]. There are also transient period between power ON and power OFF states, for which the length is determined by requirements.
Dynamic range requirements include Total power dynamic range, the difference between the maximum and the minimum transmit power of an Orthogonal Frequency Division Multiplexing (OFDM) symbol for a specified reference condition, is given by Pmax-P_RB, where P_RB is OFDM symbol (carrying PDSCH and not containing RS, PBCH or PSS/SSS) power when 1 Resource Block (RB) is transmitted. The requirement is bandwidth-dependent and the value increases with the bandwidth, starting from 7.7 dB for 1.4. MHz. Dynamic range requirements also include Resource Element (RE) Power control dynamic range, the difference between the power of an RE and the average RE power for a BS at maximum output power for a specified reference condition. The current requirement is as shown in Table 2.
TABLE 2E-UTRA BS RE power control dynamicrange (TS 36.104, Table 6.3.1.1-1)RE power controlModulation schemedynamic range (dB)used on the RE(down)(up)QPSK (PDCCH)−6+4QPSK (PDSCH)−6+316QAM (PDSCH)−3+364QAM (PDSCH)00NOTE 1:The output power per carrier shall always be less or equal to the maximum output power of the base station.
The above transmit power parameters and requirements are not signalled to the UE or exchanged over X2, although they may still be communicated among network nodes e.g. via an Operation and Maintenance (O&M) node. The parameters are verified by conformance tests, which means that changing any of the described requirements, e.g., as proposed below, may require new test configurations and new test procedures.
The following parameters may be exchanged between eNodeBs via X2, e.g., for the purpose of interference coordination. RNTP (Relative Narrowband TX Power) indication is the maximum ratio of the maximum intended Energy Per Resource Element (EPRE) of UE-specific PDSCH REs in OFDM symbols not containing RS to the maximum nominal power [TS 36.211]. RNTP is defined over a physical resource block (PRB) in a cell and may be exchange among eNodeBs via X2, e.g., to enable interference aware scheduling. RNTP can be viewed as an eNodeB “promise” to not exceed a certain power level in the specified symbols. The currently specified value range for RNTP is RNTPthresholdε{−∞,−11,−10,−9,−8,−7,−6,−5,−4,−3,−2,−1,0,+1,+2,+3}. The signalled RNTP threshold may be selectively applied for indicated by signalling PRBs (i.e., in frequency).
The transmit power of specific signals and channels in LTE also follows the general rules above. In LTE in general, the DL transmit power for specific signals/channels is typically determined in relation to the nominal power or to the Reference Signal (RS) power, e.g., cell-specific RS or UE-specific RS, as explained in more detail below.
Nominal CRS power is typically assumed when more than one antenna port are used. When CRS is transmitted from one antenna port, the transmit power may be boosted by 3 dB to use the unused power of the second port. The currently specified UE behavior is that a UE may assume [see 3GPP TS 36.213] downlink cell-specific RS EPRE is constant across the downlink system bandwidth and constant across all subframes until different cell-specific RS power information is received. The downlink reference-signal transmit power is defined as the linear average over the power contributions (in [W]) of all resource elements that carry cell-specific reference signals within the operating system bandwidth. The downlink cell-specific reference-signal EPRE can be derived from the downlink reference-signal transmit power given by the parameter referenceSignalPower provided to the UE by higher layers.
CRS are transmitted in all non-MBSFN subframes in pre-defined symbols and subcarriers and in the 1st symbol in MBSFN subframes. Effectively a 6-reuse and a 3-reuse scheme over subcarriers are used when one and more than one transmit antenna are used for CRS, respectively.
In 3GPP TS 36.214, the following E-UTRAN measurement is defined for DL RS TX power:
DefinitionDownlink reference signal transmit power is determined for a considered cell as thelinear average over the power contributions (in [W]) of the resource elements that carrycell-specific reference signals which are transmitted by the eNode B within its operatingsystem bandwidth.For DL RS TX power determination the cell-specific reference signals R0 and ifavailable R1 according TS 36.211 [3] can be used.The reference point for the DL RS TX power measurement shall be the TX antennaconnector.
The result of this measurement may be communicated by higher layers to the UE. For example, the downlink cell-specific reference-signal (CRS) Energy Per Resource Element (EPRE) can be derived from the downlink reference-signal transmit power given by the parameter referenceSignalPower signalled via RRC as a part of PDSCH configuration information which may be included in the Information Element (IE) RadioResourceConfigCommonSlB (in SIB2) and IE RadioResourceConfigCommon which are used to specify common radio resource configurations in the system information and in the mobility control information, respectively.
The referenceSignalPower may be transmitted for any serving cell, i.e. including secondary cells SCells with carrier aggregation CA.
-- ASN1STARTPDSCH-ConfigCommon ::=SEQUENCE { referenceSignalPower INTEGER (−60..50), p-b INTEGER (0..3)}PDSCH-ConfigDedicated::=SEQUENCE { p-a ENUMERATED { dB-6, dB-4dot77, dB-3, dB-1dot77, dB0, dB1, dB2, dB3}}-- ASN1STOP
MBSFN RS is used when MBMS transmissions are present in MBSFN subframes; otherwise, the MBSFN RS is not transmitted in MBSFN subframes if the latter are configured.
If CSI-RS is configured in a serving cell then a UE shall assume downlink CSI-RS EPRE is constant across the downlink system bandwidth and constant across all subframes. Zero-power CSI-RS may also be configured, and the corresponding subframe configuration may be provided separately for non-zero-power and zero-power CSI-RS, where the subframe configuration comprises offset and periodicity (5, 10, 20, 40, or 80 ms). One non-zero-power and one or more zero-power CSI-RS configurations may be configured simultaneously. The current 3GPP standard does not specify absolute or relative power specifically for CSI-RS.
The current 3GPP standard does not specify absolute or relative power specifically for DM-RS.
PRS are defined to support Observed Time Difference of Arrival OTDOA positioning. PRS are transmitted with a 9-reuse scheme over subcarriers in non-CRS symbols in positioning subframes. Several consecutive positioning subframes (1, 2, 4, or 6) are grouped into a positioning occasion. Positioning occasions have periodicity of 160, 320, 640, and 1280 ms. PRS may be transmitted over bandwidth smaller than system bandwidth or general-purpose measurement bandwidth.
The current 3GPP standard does not specify absolute or relative power for PRS, but only that it is constant within a positioning occasion. PRS muting may also be configured by the network; the network may indicate PRS muting occasions to the UE, and the UE assumes that PRS are not transmitted in the indicated occasions.
The tests are defined for PRS transmit power being the same as CRS transmit power [see TS 36.133]. Boosting of PRS power has been discussed and in fact when there are no PDSCH transmitted in positioning subframes and PRS transmit power is the same as nominal power, there is an amount of unused power in each PRS symbol which can be used to enhance hearability of PRS signals since the UEs have to detect PRS of several neighbor cells with a good geometry to enable OTDOA positioning and the neighbor cells may be located relatively far.
The current standard does not specify absolute or relative power specifically for synchronization signals PSS/SSS.
The transmit power of the downlink shared channel PDSCH is important to know for the UE. Therefore, in the current specification, it is indicated relative to the RS. Further, for higher-order MCSs, either small or no deviation from the RS transmit power is typically allowed. When UE-specific RS are not present, the ratio of PDSCH EPRE to cell-specific RS EPRE among PDSCH REs (not applicable to PDSCH REs with zero EPRE) for each OFDM symbol is denoted by UE-specific ρA or ρB parameters, depending on a pre-defined OFDM symbol index and where: ρA=δpower-offset+PA+10 log10(2)[dB] when transmit diversity is assumed for receiving PDSCH using 4 CRS antenna ports, or ρA=δpower-offset+PA [dB], otherwise; δpower-offset is 0 dB for all PDSCH transmission schemes except multi-user MIMO, PA is a UE specific parameter provided by higher layers (via RRC) and it may be {−6 dB, −4.77 dB, −3 dB, −1.77 dB}, PB is cell specific provided by higher layers (via RRC) and indicates a value of the ratio ρB/ρA which may be in the range [2/5; 5/4]. PB may also be exchanged via X2 among radio BSs. ρB is typically associated with symbols in the control region (see TS 36.211, Tables 5.2.2 and 5.2.3 for more details). For example, for one or two transmit antenna ports with normal CP, ρB is used for symbols 0 and 4 in slot 0. When UE-specific RS are present, the ratio is with respect to the UE-specific RS EPRE and in most cases it is zero. When CSI-RS are present, the assumed ratio of PDSCH EPRE to CSI-RS EPRE when UE derives CSI feedback is controlled by network-signaled parameter Pc which takes values in the range of [−8, 15] dB with 1 dB step size.